In recent years, polymer nanocomposites filled with sp2-hybridized carbons have been studied extensively. Sp′ carbons can be classified based on their dimensions and geometries. So-called zero-dimensional carbon nanostructures include buckminsterfullerenes and carbon quantum dots. One-dimensional carbon nanostructures include carbon nanotubes and nanofibers, all of which may share a linear, nanostructured morphology. Two-dimensional carbons include single-layer graphene and multilayer graphitic nanoplatelets. These are often produced from a bulk graphite precursor using liquid-phase exfoliation processes like Hummers' Method. Bulk graphitic structures such as carbon fibers or powders comprise the three-dimensional family of sp2 carbons.
While low-dimensional carbons such as nanotubes and graphene nanoplatelets possess impressive mechanical, thermal, and electrical properties, their low-dimensionality also makes them difficult to use in composite applications. Van der Waals interactions between their surfaces cause carbon nanoparticles to adhere to one another and self-assemble into disordered clusters when blended into liquid matrices (“matrix” is herein defined as a continuous liquid or solid phase surrounding the carbon nanoparticles). Carbon clusters, or “agglomerates,” reflect the tendency of low-dimensional carbons to revert to a surface energy-minimized, three-dimensional form when blended into a matrix. The effect can phase-separate the matrix and filler and degrade composite performance. To combat this phase separation, researchers have introduced “spacer” particles between graphene particles [1-3]. While spacers do not prevent agglomeration, per se, they do limit the density of the agglomerates and the occlusion of the carbon's surface area by disallowing efficient interparticle adhesion. Without spacers, nanoplatelets can agglomerate densely due to their geometry, as illustrated in FIG. 1A, which shows a cross-sectional representation of nanoplatelets and how both sides of the nanoplatelets are accessible to adhere to other nanoplatelets to form a spatially dense, low surface-area cluster.
Porous carbon nanostructures provide a promising alternative that has both two-dimensional and three-dimensional properties. Examples of such materials in the literature include ordered mesoporous carbons (OMCs) and “3D graphene.” In the case of OMC particles, researchers value the highly ordered, nanoarchitected morphologies that can be obtained due to template-directed synthesis [4]. A feature of OMCs is that the combination of their endohedral pore structure (“endohedral” herein refers to an internal cavity or surface in the carbon created by a template, while “exohedral” refers to the carbon structure's obverse surface) and their nanostructured walls allows for high specific surface areas, and their surface areas are retained so long as the endohedral surfaces are not occluded due to collapse of the endohedral pores. The spacing imposed by endohedral pores can provide a solution to the problems of nanotubes and nanoplatelets in liquids. Unfortunately, the pores are smaller than 10 nm for many OMC variants, resulting in a low pore-to-wall diametric ratio. Compared to carbons with larger endohedral cavities, OMCs can be spatially dense and difficult to impregnate and wet internally. Current research into applications for OMCs is mostly focused on adsorption and energy storage.
Some 3D nanocarbons contain larger endohedral cavities, which can theoretically be used to create a superior nanocomposite architecture. One prominent example has been obtained with aerographite, an interconnected tubular carbon network possessing nanostructured walls. As described by Garlof, et al., aerographite exhibits a “high potential for improved electrical conductivity and mechanical reinforcement of polymer nanocomposites. The incorporation of 3D nanocarbons in a polymer matrix can circumvent several drawbacks in contrast to the use of dispersed carbon nanoparticles, like agglomeration and lack of controlled network topology, hence ‘ideal’ composites can be created” [5]. Specifically, Garlof describes aerographite as a monolithic preform into which liquid epoxy resin can be infused endohedrally and exohedrally via vacuum-impregnation. The interconnectedness of the network is “the common structural motive of the Aerographite family,” according to Mecklenburg, due to its ability to support itself and, in conductive polymer nanocomposites, to serve as a highly diffuse, percolative skeleton
[6].
Like OMCs, uncollapsed aerographite specimens impose spacing between nanostructured features. However, an interconnected and continuous carbon structure may have drawbacks. Effective infusion and wetting of highly porous, continuously interconnected carbon monoliths may require low-viscosity thermosetting resins and vacuum infusion processes, and this may introduce complexity into nanocomposite fabrication-especially fabrication of thick nanocomposite components. Additionally, while a fluid dispersion can be integrated with fibrous reinforcement and fabricated using conventional tooling and manufacturing processes, continuously interconnected carbons may be less immediately practical for fabricating thick molded components, or thinly applied adhesives and coatings. Flowable liquid dispersions of discontinuous carbon nanoparticles are desirable for many nanocomposite applications.
The present invention pertains to, among other things, multiphase materials comprised of a continuous phase filled with a class of porous, 3D carbon nanostructures that offer the practical advantages of a discontinuous filler phase with a cellular morphology. These cell structures possess larger endohedral cavities than most CMK-type OMCs. Their template-directed cavity and wall morphology can allow cell particles to be made with highly regular size and shape distributions. Breaches in their cell walls may allow for infiltration of the polymer matrix material. This can result in an endohedrally impregnated cellular subunit that can self-assemble with other such subunits via van der Waals interactions into a spatially diffuse, multicellular, multiphase network with morphologically imposed phase-mixing.
For illustrative purposes, FIG. 1B shows a two-dimensional representation of a hypothetical spherical cell. The cell is a discontinuous particle. FIG. 1C is a two-dimensional representation of the spatially diffuse network created by a self-assembled cluster of these hypothetical cell particles. Because the filler is discontinuous, nanocomposites filled with this class of carbons (herein referred to collectively as “cellular carbons” or “cellular carbon structures” and individually as a “cell” or “cell structure”) can be dispersed into liquid resins, facilitating the fabrication of components requiring flowable precursors. Compared to other carbons, cellular carbons and their derivatives may provide larger, less elongated cavities, nanostructured walls, template-directed geometries and topographies, and a discontinuous form factor.